Numerous D.C. motor control circuits are known but relatively few provide forward and reverse operation from a common current source. Those that do fail to provide current flow transition in a smooth, uniform fashion, causing the controlled motor to operate in an erratic fashion. This jerkiness is unacceptable when the motor is used to drive a video camera or similar device mounting whose primary purpose is to provide a smooth panning of an objective so that the end results are pleasing to an observer.
A basic approach to provide reversible control of a D.C. motor from a single power source is illustrated in U.S. Pat. No. 3,303,403 issued to J. Bonanno for "Power Controlling Device For A Remote Toy Equipped With A Reversible Electric Motor". This device uses a polarity reversing switch combined with a pair of potentiometers. It is inefficient and generally supplies current erratically, resulting in surging motor operation.
One well known reversible D.C. motor control circuit which avoids the need for a mechanical polarity reversing switch is known as the "H" switch bridge. It is comprised of four electronic switching devices generally shown schematically as two switching devices on either side of a common load. Typical examples of the device are the Motorola "H" switch power TMOS circuit or "H" switch Darlington circuit illustrated at Pages A-90 and A-91 of Motorola's Catalog of MOSFET Circuit Data.
These "H" switch bridges are for a common load. They are adapted to allow current to flow in one direction or the other through the bridge and load in response to a means that mutually and exclusively turns on either of two pairs of the four power controlling switches.
R. Cruel in U.S. Pat. No. 3,378,740 on "Direct-Current Motor Control Circuit Employing A Parallel Damping Member" and T. Wakamiya in U.S. Pat. No. 4,210,851 on "Motor Control System" illustrate typical applications of the "H" switch bridge. They include a single transistor in each leg, thus limiting the size of the controlled motor to a relatively low powered device with little torque. They also fail to provide for a means to absorb the counter EMF created when motor speed and direction changes. This causes a jerky or erratic motor rotation transition. When a camera is driven by such a means, the results are not pleasing. Tilt and panning lacks the smooth flow in the resultant images that is usually associated with a person normally swiveling his head to view a scene.